1. Field of the Invention
The present invention relates to a proton conductor which is preferably usable, for example, as an electrolyte of a fuel cell.
2. Description of the Related Art
A fuel cell is constructed as follows. At first, an electrolyte is interposed between an anode and a cathode to provide an electrolyte-electrode joined assembly. Subsequently, the electrolyte-electrode joined assembly is interposed between a pair of separators to provide a unit cell. After that, a plurality of the unit cells are stacked to construct the fuel cell. For example, a material, which is obtained by moistening a perfluorosulfonic acid polymer membrane with liquid water, is used for the electrolyte. The electrolyte functions as a proton conductor to conduct, to the cathode, the proton which is generated by ionizing hydrogen on the anode.
The stacked unit cells are mutually tightened, for example, by means of tie rods. Therefore, the electrolyte needs to have mechanical strength such that the electrolyte is not damaged by the tightening force. Japanese Laid-Open Patent Publication No. 8-162132 suggests an electrolyte-electrode joined assembly which has a polymer solid electrolyte with an improved mechanical strength. In the case of the polymer solid electrolyte, a perfluorosulfonic acid polymer is retained in pores of drawn porous polytetrafluoroethylene (PTFE).
The proton conduction of the perfluorosulfonic acid polymer is extremely deteriorated when the polymer is dried. Therefore, a humidifier is equipped for the fuel cell. During the operation, the reaction gas, in which the humidity is raised by the steam supplied from the humidifier, is supplied. Accordingly, the electrolyte is maintained under a wet condition. In order to prevent the electrolyte from drying, the operation temperature is maintained to be 80° to 90° C. Owing to the above, the proton conduction of the electrolyte is maintained. In order to retain the operation temperature at 80° to 90° C., a cooling medium generally flows into the fuel cell.
The fuel cell is further equipped with a heater so that the steam for raising the humidity is not frozen when the outside air temperature is less than 0° C., for the following reason. That is, if the steam is frozen, the proton conductivity of the electrolyte is lowered. The fuel cell is heated by the heater, and the operation is started after the temperature of the fuel cell is higher than the freezing temperature of the steam.
According to the explanation as described above, it can be understood that the humidifier, the cooling system for allowing the cooling medium to flow, and the heater are required for the fuel cell in which the electrolyte is composed of the perfluorosulfonic acid polymer or the fuel cell in which the perfluorosulfonic acid polymer is retained in the pores of the electrolyte as a porous member. However, in the case of the fuel cell constructed as described above, the fuel cell consequently has a large size. Further, the operation cost is expensive for the fuel cell.